You've come to the right guy. I had really bad p.f. in college, when I was running around 100 mpw and it got worse when doing intervals. I could hardly walk in the mornings until I had soaked my feet in warm water and epsom salts.
The cortisone shots worked. They hurt like hell; but worked. I rested for 5 days minimum, after the shots and a week another time I got them. The pain was gone!
However, at some point the strain and pain could come back unless you correct the biomechanic problems that strained the plantar ligiments. Mine was high arches and flimsy racing flats. I eventually got custom built orthotics which kept the p.f. from ever coming back and I ran many 100 mile plus weeks for many years after that, did intervals, etc. But, p.f. did cripple me for about 2 years in college.